1. Field of the Invention
This invention relates to an apparatus for determining the purity of a substance by monitoring the change in a temperature dependent physical property of the substance, and more particularly, to an apparatus for monitoring the depression of the freezing point of the substance.
2. Background of the Invention
Among the most important measurements made in the chemical industry is the determination of the purity of substances such as reagent grade chemicals. Many of these chemicals are required by custom and/or law to be labeled on the basis of the purity of the product. An accurate way of determining the purity of a substance is by the measurement of the depression of the freezing point of the substance caused by the presence of the impurities. Raoult's Law states that the inclusion of impurities within a substance results in a freezing point depression and a boiling point elevation of the impure substance compared to the pure substance. The magnitude of these changes is dependent on the concentration of the included impurities. The theory of Raoult's Law is explained in detail in my paper entitled "Automatic Purity Analysis by Freezing Point Measurement" presented at the 22nd International Instrumentation Symposium, May 1976, in San Francisco, Calif. The aforementioned paper is hereby incorporated by reference into the instance specification.
Probably the most common method used for determining the purity of reagent grade chemicals is the ASTM D 1015 method. Unfortunately, this method is not practical for a continuous flow system due to a prohibitive time lag between the time the samples are obtained and the time required to accurately determine the purity. Accordingly, this method is more applicable to a batch process rather than a continuous flow system.
Therefore it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art measurements and provides an improvement which is a significant contribution to the advancement of the art of purity analysis.
Another object of this invention is to provide an apparatus for measuring the concentration of impurities within a substance by the measurement of the reduction in the freezing point of the substance which is readily applicable to a continuous flow chemical process and provides several accurate determinations of purity per hour.
Another object of this invention is to provide an apparatus for measuring the concentration of impurities within a substance by the measurement of the reduction in the freezing point of the substance incorporating a cell having an input and an output conduit for receiving and discharging the substance and including temperature variation means such as a solid state thermoelectric device for varying the temperature of the substance within the cell.
Another object of this invention is to provide an apparatus for measuring the concentration of impurities within a substance by the measurement of the reduction in freezing point of the substance including a thermistor probe established within the cell for sensing the temperature of the substance and a novel amplifier module for providing a control output capable of controlling the temperature of the substance and a precision output for indicating the precise concentration of impurities within the substance.
Another object of this invention is to provide an apparatus for measuring the concentration of impurities within a substance by the measurement of the reduction in the freezing point of the substance wherein the control means may be programmed for receiving a new sample of the substance into the apparatus at pre-established intervals of time for providing plural tests of the substance for indicating the change in concentration of impurities within the substance as a function of time.
Another object of this invention is to provide an apparatus for measuring the concentration of impurities within a substance by the measurement of the reduction in the freezing point of the substance including a thermoelectric device interposed between the substance container and a heat sink for transferring heat from the substance in the container to a circulating cooling liquid within the heat sink.
Another object of this invention is to provide an apparatus for measuring the concentration of impurities within a substance by the measurement of the reduction in the freezing point of the substance which provides an output which is compatible with computers, recorders and the like.
Other objects and a fuller understanding of this invention may be had by referring to the summary of the invention, the description and the claims, taken in conjunction with the accompanying drawings.